feat: phase 1 core completeness (hashers, prefetch, cancel, focus triggers)

[matheuseabra]

Implements all Phase 1 tasks from docs/roadmap.md.

Behavior changed

• Custom key hashers: Query.make({ hash }) overrides hashing for fetch; default stays JSON.stringify. Raw-key ops (getData, setData, invalidate, cancel) keep structural identity. Unhashable keys fail typed with new KeyHashError, never throw.
• Query.notifyFocus() / Query.notifyReconnect() revalidate entries whose queries opt in via refetchOnFocus / refetchOnReconnect, in background daemons.
• Query.prefetch(query, ...args) warms the cache without awaiting data (fire-and-forget daemon).
• Query.cancel(key) interrupts the in-flight winner fiber for a key and clears the slot; idle/missing keys are no-ops.
• Single-flight reworked: winner forks the request into a supervised child; child taps cache on success; winner funnels every outcome through the shared Deferred in an onExit finalizer, so joiners are always released.

API / cache-semantics notes (breaking within 0.x)

• invalidate, setData, getData, cancel now carry KeyHashError in the error channel (was never); fetch is E | KeyHashError.
• MutationDefinition gains an E2 = never parameter so onSuccess hooks may fail typed (e.g. fallible invalidation).
• Trigger revalidation only has QueryService in scope: queries needing more fail silently in the daemon without touching cached data.

Tests

• 9 new behavior tests (30 total): custom hasher sharing, typed hash failures on fetch and manual reads, focus/reconnect revalidation and no-ops, prefetch warming without awaiting, cancel-interrupt with slot reuse, cancel no-ops.
• pnpm check green: typecheck, lint, 30/30 tests, 100% line/function and 97.7% branch coverage (thresholds hold), build clean.

Spec links

• docs/roadmap.md Phase 1 boxes checked; docs/spec.md and docs/architecture.md updated (including interruption semantics per operation).

No version bump: release as 0.2.0-beta.0 happens separately via tag.